1. Field of the Invention
This invention relates to the field of metal plating and stripping and particularly to an improved composition permitting accelerated removal of electroless deposited nickel from ferrous, titanium and aluminum substrates.
Electroless nickel has found application in many fields because of the ease with which it may be applied to various metals while providing effective corrosion resistance and desirable engineering properties at microthickness levels. In recent years, procedures for applying the nickel coatings have advanced to a point where the layer of nickel admirably serves its purpose of protecting the surface of the metal substrate but removal has become a vexatious and troublesome problem.
2. Description of the Prior Art
Solutions for stripping electroless applied nickel for many years were primarily based on inclusion of cyanide compounds as oxidizing and electrolytic agents in the presence of a caustic compound. Usually, sodium cyanide was dissolved in a water solution containing potassium or sodium hydroxide. Efforts were made to enhance functionality of the stripping solution by incorporation of additives. One of the most effective in this respect was found to be sodium metanitrobenzene sulfonate. Although this composition worked effectively to remove the nickel coating if the part to be stripped was allowed to remain in the bath for a sufficient period of time (often as long as several days to a month), and the solution maintained at an elevated temperature of about 120.degree. F. to about 130.degree. F., disposal of the stripping bath presented real problems not only because of the difficulty of safely disposing of the cyanide solution, but equally as importantly, the amount of nickel present in the bath presented real challenges to disposal service organizations. Furthermore, the time of removal inherently increased the cost of the stripping operation.
Although electroless applied nickel has advantages as pointed out, the use of such coatings has been found to present problems if need arises for removal thereof because of the high cost of stripping. In recent years, it has been found that removal can cost from two to ten times the amount charged for application to the metal parts.
Efforts to solve the environmental problems relating to disposal of cyanide stripping solutions which include relatively large concentrations of nickel therein have been directed primarily to provision of stripping agents which do not include cyanide as a component part. Efforts to increase the effectiveness of caustic solutions have for the most part involved the incorporation of amine-based additives such as EDTA or similar amines along with additional stripping enhancers. However, these stripping solutions have been found to have a relatively short life since they can only absorb a limited amount of nickel and disposal of the strippers in accordance with environmental regulations has not been totally solved as of yet.